1. Field of the Invention
The invention relates to magnetic tunnel junction sensors typically used in magnetic disk drives and MRAM memory devices; and, more specifically to self-pinned magnetic tunnel junction sensors.
2. Description of the Background Art
Disk drives using magnetic recording of digital information store most of the information in contemporary computer systems. A disk drive has at least one rotating disk with discrete concentric tracks of data. Each disk drive also has at least one recording head typically having a separate write element and read element for writing and reading the data on the tracks. The recording head is constructed on a slider and the slider is attached to a suspension. The combination of the recording head, slider, and suspension is called a head gimbal assembly. In addition, there is an actuator which positions the recording head over the specific track of interest. The actuator first rotates to seek the track of interest and after positioning the recording head over the track, maintains the recording head in close registration to that track. The disk in a disk drive has a substrate and a magnetic layer formed over the substrate for magnetic recording. The slider carrying the recording head has a disk facing surface upon which an air bearing is constructed. The air bearing allows the slider to float on a cushion of air and to be positioned close to the disk surface. Alternatively, the slider surface facing the disk can be adapted for partial or continuous contact with the disk.
The read element in most contemporary disk drives is a current-in-plane (CIP) spin valve sensor. A spin valve sensor is a sandwich of layers including a ferromagnetic pinned layer, a nonmagnetic electrically conducting layer, and a ferromagnetic free layer. The resistance of the spin valve sensor changes with respect to the direction and magnitude of an applied magnetic field such as the field from a written magnetic transition on a disk. To detect the change in resistance, sense current is passed through the sensor. The sense current is passed along the plane defined by the sandwich of layers and accordingly the sensor is referred to as a current-in-plane (CIP) sensor. Generally, any other layers such as the substrate or seed layers associated with a CIP spin valve sensor should be formed from an electrically insulating material to avoid current shunting and subsequent loss of sensitivity of the sensor.
Current-perpendicular-to-plane (CPP) magnetic tunnel junction sensors are more sensitive than CIP spin valve sensors. Each new generation of disk drives generally has a higher density of recorded information. As the size of recorded bits of information on the disk is reduced, the sensitivity of the read element becomes more important. Thus, CPP magnetic tunnel junction sensors are desirable for high density magnetic recording applications. A CPP magnetic tunnel junction sensor operates according to a different physical principle than a CIP spin valve sensor. A CPP magnetic tunnel junction sensor includes a sandwich of layers including a pinned layer and free layer. The pinned layer is separated from the free layer by a thin nonmagnetic, insulating layer. Sense current is applied perpendicular to the plane of the sandwich of layers. The electrons of the sense current pass through the thin insulating layer according to a quantum mechanical effect called tunneling. The magnitude of the tunneling current depends on the direction of the magnetization in the free layer relative to the direction of the net magnetization in the pinned layer.
The direction of net magnetization in the pinned layer is typically held in place with an adjacent antiferromagnetic layer which is exchange coupled with the pinned layer. The antiferromagnetic layer is relatively thick and requires initialization to be effective. The initialization usually requires the antiferromagnetic layer be annealed at an elevated temperature in the presence of a magnetic field. Some tunnel junction sensors employ a second antiferromagnetic layer indirectly coupled to the free layer for magnetic bias stabilization of the free layer. This second antiferromagnetic layer also requires initialization and the required direction of the second magnetic annealing field is orthogonal to the direction of the magnetic annealing field used for the antiferromagnetic layer coupled to the pinned layer. Thus two initialization steps are required for a conventional magnetic tunnel junction sensor. Because of the complexity of using two orthogonal initialization magnetic fields, a conventional magnetic tunnel junction sensor with two antiferromagnetic layers is difficult to manufacture. All the layers in a CPP tunnel junction sensor except for the insulating layer between the pinned layer and the free layer are necessarily electrically conductive.
A CPP magnetic tunnel junction sensor is constructed between two magnetic shields in order to control the spatial resolution along the direction of the recorded track on the disk. As the gap between the two magnetic shields is reduced, the spatial resolution improves and the recording head is better able to support higher recorded densities. Thus it is very desirable for the sandwich of layers which comprise the sensor be as thin as possible. Thus what is needed is a CPP magnetic tunnel junction sensor which is very thin and easy to manufacture.